This invention relates to the purification of hydrocarbon-containing gaseous mixtures. More particularly, this invention relates to the purification of hydrocarbon-containing gaseous mixtures which includes both carbon dioxide and one or more sulfur-containing compounds, such as hydrogen sulfide, alkyl mercaptans, carbonyl sulfide and the like.
In recent years, natural gas producers have become more and more dependent on "sour" gas sources than ever before. As used herein, a "sour" gas is defined as a hydrocarbon-containing gas which includes hydrogen sulfide and/or alkyl mercaptans. Such sulfur-containing compounds should be removed in order to obtain high quality, fuel grade natural gas and liquified petroleum gas products. In particular, natural gas is often used as a feed material for the production of ammonia. In this situation, it is advantageous to reduce the hydrogen sulfide and alkyl mercaptan contents of the natural gas to avoid problems, in the ammonia production process. In addition, such sour gases often include substantial amounts of carbon dioxide, and possibly carbonyl sulfide. The carbon dioxide in such hydrocarbon-containing gases acts as a diluent and is advantageously removed prior to using the gas.
Thus, hydrocarbon-containing gases which include both carbon dioxide and sulfur-containing compounds at undesirably high levels must be treated to reduce or eliminate the presence of such components. Heretofore it has been very difficult to provide an economically effective process to reduce both the concentrations of carbon dioxide and mixtures of sulfur-containing compounds in such hydrocarbon-containing gases.
Prior art processing involving the use of alkaline solutions, such as aqueous solutions of potassium carbonate and alkanolamines, are effective to reduce the carbon dioxide concentration in such gases to an acceptably low level. In addition, such absorption technology can reduce the concentrations of hydrogen sulfide and carbonyl sulfide to acceptably low levels. However, such technology does not reduce the level of alkyl mercaptans sufficiently to meet high quality, e.g., ammonia feedstock, specifications. Other technologies, in particular adsorption technologies, have been found to be useful to reduce alkyl mercaptan concentrations to acceptably low levels. However, such adsorption technologies do not economically reduce the level of carbon dioxide in the gas. Moreover, certain adsorbents themselves actually produce carbonyl sulfide during the adsorption of other sulfur-containing compounds.
A typical adsorption process comprises passing a sulfur-containing, hydrocarbon-containing gaseous stream through a bed of a molecular sieve adsorbent having a pore size large enough to adsorb the sulfur impurities, and removing the non-adsorbed effluent hydrocarbon until a desired degree of loading of the adsorbent with sulfur-containing impurities is obtained. Thereafter, the adsorbent mass is regenerated by desorbing the sulfur-containing compounds from the spent sorbent.
The adsorbent regenerating operation is conventionally a thermal swing, or combined thermal and pressure swing, operation. When treating a hydrocarbon-containing gas with an adsorbent, such as crystalline zeolitic molecular sieves, a purge gas is provided to regenerate the sulfur compound laden adsorbent. Generally a slipstream of product gas produced while the adsorbent is operated in the adsorption mode is utilized as the desorption gas. The utilization of this product gas for regeneration purposes disadvantageously reduces the final product gas yield. In addition, the regeneration effluent, i.e., the purged gas leaving the adsorbent bed with the sulfur compounds, must be discarded and is often used as fuel, flared or otherwise disposed of at little or no value.
A need exists to provide a process for efficiently and effectively removing carbon dioxide, hydrogen sulfide and alkyl mercaptans from hydrocarbon-containing gaseous streams, particularly when such components are present in such streams in relatively high concentrations.